Under mattresses for slatted beds are of two kinds, either with slats having adjustable rigidity, or with slats having fixed rigidity.
For slatted beds with adjustable rigidity, the rigidity of the slat itself is modified empirically by the user trying it. Because of the location of the slat rigidity adjusting means, this method is impractical. It requires multiple attempts and much trial and error to succeed in satisfying the comfort of the user.